Why don't all siblicidal eagles lay insurance eggs? The egg quality hypothesis

Several species of birds lay second eggs that are eliminatedby the siblicidal behavior of the first-hatched chick. A widelyaccepted explanation for the occurrence of these second eggsis insurance against complete nest failure. However, if insuranceis seen as an important breeding strategy for two-egg (c/2)layers, the question arises why single-egg species do not layinsurance eggs. The insurance-egg hypothesis predicts that extraeggs should occur where hatch failure is not trivial, whichmay be particularly prevalent in dense populations. Neitherprediction was supported for siblicidal l Wahlberg's eaglesAquila wahlberge Neither could food constraints or allometricrelationships explain the small one-egg clutch (c/1) of thisspecies Instead, clutch size was experimentally shown to berelated to optimal brood size: parents given two young wereunable to rear them, and subsequent breeding opportunities weresignificantly curtailed. Since clutch and brood size are similarlyrelated in c/2 eagles, insurance may be an exaptation of thesecond egg. One-egg spedes, however, appear to trade second(insurance) eggs for large, high-quality eggs, which enhancehatchability and chick viability. This was borne out by comparisonof the world's c/1 eagles, which lay significantly (p<.01)larger eggs than c/2 eagles of the same body size. Large Wahlberg'seagle eggs also showed significantly (p=.02) greater hatchabilitythan small eggs, and other studies show enhanced survival/qualityfor chicks from large eggs. Because only longer-lived eaglestraded two eggs for single, large eggs, this is consistent withthe idea of selection for offspring quality in long-lived species.I condude that higher hatchability of single, large eggs decreasesthe need for an insurance egg and simultaneously enhances viabilityof resultant chicks in sibliddal eagles and possibly sulids.     

Why do Luehdorfia butterflies lay eggs in clusters?

Luehdorfia butterflies lay eggs in clusters. Clones of their host plants (Asiasarum and Heterotropa) are distributed pacthily among the understory of deciduous forests. Groups of Luehdorfia larvae often exhaust the clones and may wander over the forest floor seeking new clones. The highest mortality observed is during this wandering period. To elucidate why Luehdorfia butterflies lay eggs in clusters, a simulation experiment was made for hypothetical populations which lay eggs in clusters or singly. Field data on larval mortality, consumption, density of host clones and leaf weight for Luehdorfia japonica were incorporated into the model. The predictions of the simulation were: (1) When the egg density is low, the single egg type could leave many more pupae than the egg clustering type, but when the egg density is high, the former might leave smaller number of pupae than the latter; and (2) There are optimal sizes of egg clusters for different egg densities and the optimal size becomes larger as the egg density increased.     

The problem with zeroes: why don't drosophilids lay eggs?

Summary We present some data on drosophilid oviposition and analyse the distribution of egg numbers over patches using an iterated negative binomial. This suggests that there are three different reasons for empty patches; patches are not found, patches are not suitable, or females are not able to lay eggs. This leads to five categories of site which can be disentangled using the iterated negative binomial. Since empty patches have important consequences for population dynamics and coexistence, this analysis will highlight how microscopic processes influence macroscopic behaviour in population biology.     

Why do female lizards lay their eggs in communal nests?

1. In many reptile species, females oviposit communally (i.e. many clutches are laid within the same nest). This behaviour might result from constraint (scarcity of nest-sites offering suitable incubation conditions) or adaptation (direct fitness benefits accruing from the proximity of other eggs). 2. To test between these alternatives, we gathered field and laboratory data on montane scincid lizards Bassiana duperreyi from south-eastern Australia. Our data support the adaptationist hypothesis. 3. In the field, communal vs. solitary clutches were laid in similar sites, and the relative frequency of communal nesting was not predictable from nest-site availability. Thermal regimes for incubation did not differ between communal vs. solitary nests, nor between eggs at the core vs. periphery of a communal nest. In the laboratory, females selectively oviposited beside existing eggs rather than in otherwise identical potential nesting sites. 4. From cycling-temperature incubation in the laboratory, eggs incubated within a cluster of other eggs took up less water, but produced hatchlings that were larger and faster-running than were hatchlings from eggs incubated alone. 5. Hydric modifications of incubation conditions within a cluster of tightly packed eggs thus may provide a direct fitness benefit to communal oviposition.     

Can birds count eggs in their nests?

Sensory mechanisms controlling avian clutch size have diversified into distinct types, according to the nature of the input that is used to disrupt the growth of ovarian follicles and hence halt egg‐laying. In an article on brood parasitism, Lyon (2003) claimed that female American Coots Fulica americana can reduce their clutch size on the basis of visual cues in response to eggs laid in their nests by other females; in this species, therefore, egg counting would be used to control clutch size. After a close examination of the physiological determination of clutch size in American Coots, I show that seven of 17 parasitized clutches were smaller than the range controlled through the mechanism using an input to disrupt follicular growth (7–10 eggs per clutch). My reanalysis suggests that American Coots are incapable of adjusting clutch size via counting and re‐asserts that a species that can count eggs has yet to be found among birds that rely upon their own body heat for incubation.     

Why are female birds ornamented?

Sexual selection is now widely accepted as the main evolutionary explanation of extravagant male ornaments. By contrast, ornaments occurring in females have received little attention and often have been considered as nonadaptive, correlated effects of selection on males. However, recent comparative evidence suggests that female ornaments have evolved quite independently of male showiness. Also, new theoretical models predict that both male mate choice and female contest competition will occur under certain circumstances. This is supported by recent experimental studies. Thus, selection acting on females might be a widespread cause of female ornaments.     

Why don't we have a schistosomiasis vaccine?

Over the past 30 years there has been a concerted effort to understand host immune responses to schistosomes, with the ultimate aim of producing a vaccine for human use. In this issue, Bergquist and Colley, in summarizing recent meetings in Cairo, provide a detailed appraisal of progress towards that goal. It seems an appropriate time to ask why, with reference to Schistosoma mansoni, the development of a vaccine has proved so difficult. This question is explored here by Alan Wilson and Pat Coulson.     

Why are birds’ eggs speckled?

Birds are unique in laying eggs with pigmented shells, but for most species (e.g. most passerines, which lay white eggs speckled with reddish spots of protoporphyrin) the pigments’ function is unknown. We studied a bird population at a geologically variable site, and considered a hitherto untested hypothesis: that protoporphyrin pigments might compensate for reduced eggshell‐thickness (caused partly by calcium deficiency), which is known to reduce eggshell‐strength and increase eggshell‐permeability. We found that pigment spots specifically demarcated thinner areas of shell, with darker spots marking yet thinner shell than paler spots. Variation in pigmentation was thus associated with variation in shell thickness both within and between clutches, so accounting for the eggshell's characteristic spot patterns. Geological variability at this site has resulted in a great range of calcium availability and, as predicted by the hypothesis, variation in calcium availability was found to affect between‐clutch variation in both eggshell‐mass (+) and pigmentation characteristics (?). We suggest a physiological mechanism and some important implications of these findings.     

Why don't we get more cancer? A proposed role of the microenvironment in restraining cancer progression

Tumors are like new organs and are made of multiple cell types and components. The tumor competes with the normal microenvironment to overcome antitumorigenic pressures. Before that battle is won, the tumor may exist within the organ unnoticed by the host, referred to as 'occult cancer'. We review how normal tissue homeostasis and architecture inhibit progression of cancer and how changes in the microenvironment can shift the balance of these signals to the procancerous state. We also include a discussion of how this information is being tailored for clinical use.     

Why don't we use vitamin E in dermatology?

OBJECTIVE: To review the possible uses of topical and systemic tocopherols as therapy for skin conditions in light of the widespread use of vitamin E by patients. DATA SOURCES: Index Medicus was searched for articles published from 1922 (when vitamin E was discovered) to 1966 (the beginning of MEDLINE). MEDLINE was searched for articles in English and French on vitamin E or tocopherol in relation to dermatology. Additional original articles were identified from the reference lists of the review articles. STUDY SELECTION: Only well-designed controlled studies were accepted; anecdotes and open studies are cited for completeness and as direction for future research. DATA SYNTHESIS: There was some weak or conflicting evidence that vitamin E is of value in yellow nail syndrome, vibration disease, epidermolysis bullosa, cancer prevention, claudication, cutaneous ulcers, and collagen synthesis and wound healing. It was of no use in atopic dermatitis, dermatitis herpetiformis, psoriasis, subcorneal pustular dermatosis, porphyrias and skin damage induced by ultraviolet light. CONCLUSIONS: After 44 years of research there is still scant proof of vitamin E''s effectiveness in treating certain dermatologic conditions. Further research in well-designed controlled trials is needed to clarify vitamin E''s role.     

When should Common Cuckoos Cuculus canorus lay their eggs in host nests?

Capsule Brood parasitic Common Cuckoo Cuculus canorus chicks hatch earlier than the nestlings of their Great Reed Warbler Acrocephalus arundinaceus hosts, but hatching priority is less consistent when Cuckoo eggs are laid after the onset of host incubation.

Aim To reveal by field observations what the optimal stage is for Cuckoos to lay their eggs in relation to the host laying cycle to ensure prior hatching of the parasitic chicks.

Methods We monitored the hatching of Cuckoo chicks in relation to the hosts’ laying stage at which the Cuckoo eggs appeared and also monitored host incubation behaviour.

Results Great Reed Warblers incubated more on day 5 after the host's onset of laying relative to day 3. All Cuckoo eggs hatched earlier than hosts when they were laid prior to the onset of host incubation (day 4). Cuckoo eggs also maintained hatching priority in about 2/3 of the nests when laid on days 5–6.

Conclusions Most Cuckoo eggs are laid prior to the onset of host incubation and this, together with other adaptive mechanisms, ensures the prior hatching of Cuckoo eggs. Cuckoo eggs laid after the onset of incubation lose the advantage of prior hatching in approximately 30% of nests.     

Why is mimicry in cuckoo eggs sometimes so poor?

I propose that the existence of imperfect adaptations (e.g. egg mimicry) in brood parasites and their hosts (e.g. discrimination abilities) could reflect age-dependent territory and nest-site selection patterns of the host. Studies of various passerines indicate that (1) older breeders tend to occupy nest sites of higher quality than do young birds (ideal despotic distribution resulting from interference competition), (2) nest-site selection affects the risk of parasitism in various habitats, (3) egg recognition in passerines has a strong learning component (therefore naive breeders tend to accept whereas older birds tend to reject parasitic eggs). Because young naive birds, who tend to accept parasitic eggs, usually breed in low-quality areas where they are frequently parasitised, while old experienced birds, who tend to reject parasitic eggs, breed in high-quality areas where they are rarely parasitised, the distribution of acceptors and rejecters with respect to the risk of parasitism is non-random, i.e. populations of some host species may consist of heavily parasitised acceptors and weakly parasitised rejecters. Therefore, the selection pressure exerted by the host on the parasite should be weaker than if brood parasitism was randomly distributed among naive and experienced breeders and affect adaptations such as egg mimicry. This could explain the existence of imperfect adaptations in some brood parasite-host systems.     

Why do Red-winged Blackbrids accept eggs of Brown-headed Cowbirds?

Summary Avian brood parasites usually severely depress the reproductive success of their hosts, yet many host species, including those presumably capable of ejecting parasitic eggs, accept them. Female, brood-parasitic, Brown-headed Cowbirds typically remove a host egg when they lay their own and damage some host eggs in the process of ejecting a host egg. Data from a field study of Red-winged Blackbirds show that these costs, which cannot be avoided by ejecting the parasitic egg, account for some of the reproductive losses attributable to parasitism, but part is due to the presence of the cowbird egg in the nest. To assess whether ejection would be favoured under current circumstances, the probable damage a female Redwing could cause to her own eggs by attempting to eject a cowbird egg needs to be determined.     

Why do female migratory birds arrive later than males?

1. In migratory birds males tend to arrive first on breeding grounds, except in sex-role reversed species. The two most common explanations are the rank advantage hypothesis, in which male-male competition for breeding sites drives stronger selection for early arrival in males than females, and the mate opportunity hypothesis, which relies on sexual selection, as early arrival improves prospects of mate acquisition more for males than for females. 2. To date, theoretical work has focused on selection for early arrival within a single sex, usually male. However, if fitness depends on territory quality, selection for early arrival should operate on both sexes. Here we use two independent modelling approaches to explore the evolution of protandry (male-first arrival) and protogyny (female-first arrival) under the rank advantage and mate opportunity hypotheses. 3. The rank advantage hypothesis, when operating alone, fails to produce consistent patterns of protandry, despite our assumption that males must occupy territories before females. This is because an individual of either sex benefits if it out-competes same-sex competitors. Rather than promoting protandry, the rank advantage mechanism can sometimes result in protogyny. Female-female competition is stronger than male-male competition early in the season, if females compete for a resource (territories occupied by males) that is initially less common than the resource of interest to males (unoccupied territories). 4. Our results support the mate opportunity hypothesis as an explanation of why protandry is the norm in migratory systems. Male-biased adult sex ratios and high levels of sperm competition (modelled as extra-pair young: EPY) both produce protandry as a result of sexual selection. Protogyny is only observed in our models with female-biased sex ratios and low EPY production. 5. We also show that the effects of sex ratio biases are much stronger than those of EPY production, explore the evidence for sex ratio biases and extra-pair paternity in migratory species and suggest future research directions.     

Why do mountains support so many species of birds?

Although topographic complexity is often associated with high bird diversity at broad geographic scales, little is known about the relative contributions of geomorphologic heterogeneity and altitudinal climatic gradients found in mountains. We analysed the birds in the western mountains of the New World to examine the two‐fold effect of topography on species richness patterns, using two grains at the intercontinental extent and within temperate and tropical latitudes. Birds were also classified as montane or lowland, based on their overall distributions in the hemisphere. We estimated range in temperature within each cell and the standard deviation in elevation (topographic roughness) based on all pixels within each cell. We used path analysis to test for the independent effects of topographic roughness and temperature range on species richness while controlling for the collinearity between topographic variables. At the intercontinental extent, actual evapotranspiration (AET) was the primary driver of species richness patterns of all species taken together and of lowland species considered separately. In contrast, within‐cell temperature gradients strongly influenced the richness of montane species. Regional partitioning of the data also suggested that range in temperature either by itself or acting in combination with AET had the strongest “effect” on montane bird species richness everywhere. Topographic roughness had weaker “effects” on richness variation throughout, although its positive relationship with richness increased slightly in the tropics. We conclude that bird diversity gradients in mountains primarily reflect local climatic gradients. Widespread (lowland) species and narrow‐ranged (montane) species respond similarly to changes in the environment, differing only in that the richness of lowland species correlates better with broad‐scale climatic effects (AET), whereas mesoscale climatic variation accounts for richness patterns of montane species. Thus, latitudinal and altitudinal gradients in species richness can be explained through similar climatic‐based processes, as has long been argued.